Thermal insulating cellulose/wood foam for passive radiant cooling

Passive cooling permits thermal management of near-zero energy consumption and low CO₂ emissions. Herein, cellulose/wood chip composite foam (CWF) with anisotropic porous structure was prepared via freeze-casting strategy. The CWF displayed an average reflectance of up to 95.2 % in the UV to NIR light range (0.2–2.5 μm), as well as with an average emissivity of 94.8 % in the atmospheric transparent window (8–13 μm). The theoretical cooling power reaches approximately 130 W/m², thus making it suitable for applications in fields such as passive cooling building materials. In addition, hydrophobic coating was further applied to the CWF, which not only endowed the CWF with moisture resistance, but also enhanced the reflectivity. This novel CWF composed of natural polymer and forest wastes will pave the way for smart passive coolers of high efficiency, sustainability and low cost.